Blood filters are widely used in extracorporeal blood flow circuits, such as those employed in hemodialysis treatments or cardiopulmonary bypass operations (e.g. open heart surgery). These filters are typically disposable, i.e. not resterilized and re-used, and are thus manufactured in mass production from inexpensive materials. In cardiopulmonary bypass circuits, blood filters are usually included both upstream and downstream of a blood oxygenator. An arterial blood filter, located downstream from the oxygenator, is intended to serve a critically important safety function by removing any solid or gaseous emboli, particles, bubbles, etc., that may for example have escaped through the oxygenator or been generated by cavitation behind a pump, from the arterialized blood before it is returned to the patient. Failure to effectively remove such emboli, particles, bubbles, etc., can obviously have disastrous consequences.
As an additional safety factor, an arterial blood filter positioned downstream from a pump must have a very high resistance to rupture under an excessive internal pressurization caused for example by an unexpected blockage of the return line to the patient. In a disposable arterial filter, the high resistance to rupture must be accomplished without markedly raising the cost of manufacture.
One known type of arterial blood filter (see e.g. U.S. Pat. Nos. 3,701,433 and 3,939,078) comprises a hollow tubular housing, an upwardly-extending perforated tubular core concentrically disposed within the housing and surrounded by a cylindrical filter element, e.g. a pleated layer or array of layers wrapped into a cylindrical configuration, a filter element cap covering the upper ends of the perforated core and filter element, a gas vent in the top wall of the housing, a blood inlet in communication with the space between the filter element and the side wall of the housing, and a blood outlet in communication with the space within the perforated core. The flow of blood through the cylindrical filter element is substantially radial, from the outside of the cylindrical element to the inside thereof. Although this known type of arterial blood filter has been used for many years with considerable benefit to mankind, it is nevertheless in need of improvement. The input blood upstream of the filter element tends to develop regions of churning flow, which interfere with the orderly passage of gaseous emboli and bubbles to the vent. As a result, one must rely excessively upon the layer in the filter element having the smallest pore size, e.g. a woven filter screen, to prevent the passage of gaseous emboli and bubbles through the filter element and to the patient. Consequently, the probability of such passage occurring is higher than if a smooth orderly gas venting were provided. Furthermore, the development of regions of churning flow may give rise to excessive destruction of blood components.